The specific surface area of zirconia units conventionally used as catalyst supports is at most about 100 m2/g at 400° C. In addition, those having a greater specific surface area are typically amorphous without having a crystal structure. Consequently, even if a zirconia unit is used as a catalyst support, as a result of the specific surface area decreasing at high temperatures of 400° C. or higher, it is not possible to obtain stable performance at high temperatures. Thus, it is necessary to further improve heat resistance in order to use as a catalyst support.
In contrast, zirconia-ceria compositions composed of zirconium oxide and cerium oxide are typically able to secure a comparatively large specific surface area even at a high temperature of 1000° C., and have heat resistance superior to that of zirconia and the like when used as a catalyst.
At present, there have been numerous reports of attempts to further improve heat resistance by adding rare earth metal oxides or alkaline earth metal oxides and the like other than ceria to zirconia-ceria compositions.
In actuality, however, since the important function of a co-catalyst, in addition to heat resistance, is the oxidation-reduction potential of ceria in an oxidation-reduction atmosphere, it is becoming an indispensable characteristic for improving catalyst performance.
Japanese Patent No. 3490456 describes a “composition having for a base material thereof a zirconium oxide containing cerium oxide and at least one type of doping element; wherein, the composition is provided in the form of a single phase of zirconium oxide crystallized into a cubic system or tetragonal system, the cerium oxide and doping element contained therein is present as a solid solution, and the composition has a specific surface area of 25 to 51 m2/g after firing for 6 hours at 1000° C.”.
In addition, Japanese Patent Application Publication No. H10-194742 describes a “zirconium-cerium-based mixed oxide obtained by firing at 500 to 1000° C.; wherein, the mixed oxide contains zirconium and cerium, the mixing ratio of the zirconium and cerium as zirconium oxide and cerium (IV) oxide is 51 to 95:49 to 5 as the weigh ratio thereof, the mixed oxide demonstrates a specific surface area after the firing for 6 hours at 500 to 1000° C. of at least 50 m2/g, and maintains a specific surface area of at least 20 m2/g after heating for 6 hours at 1100° C.”.
However, there is no description regarding the reduction rate of ceria in Japanese Patent No. 3490456 and Japanese Patent Application Publication No. H10-194742.
On the other hand, Japanese Patent No. 3623517 describes a “composition comprising cerium oxide and zirconium oxide of at least one cerium/zirconium atomic ratio; wherein the composition demonstrates a specific surface area of at least 35 m2/g after firing for 6 hours at 900° C., and demonstrates an oxygen storage capacity of 1.5 ml/g at 400° C.”.
However, the ceria reduction rate is described in the examples as being a low value of a maximum of about 12%.
Moreover, Published Japanese Translation No. 2006-513973 of a PCT International Publication describes a “composition containing zirconium oxide and cerium oxide having a ratio of zirconium oxide of at least 50% by weight; wherein, the maximum reductibility temperature after firing for 6 hours at 500° C. is 500° C. or lower, the specific surface area is 40 m2/g or more, and the composition is in the form of a tetragonal system phase.”
However, although reduction rate of ceria is described as being 80% in the examples, the specific surface area of 38 m2/g after heat treatment for 6 hours at 1000° C. is not satisfactory in terms of heat resistance.